Monthly Archives: June, 2012

Mild Curiosity



High up in Ely Cathedral…


Paper is a fun drawing app for the iPad, and a flattering one: it can make you look like a better artist than you really are.

A few quick sketches, done in bed one night…


Favourite quote of the day

One man’s fish is another man’s ‘poisson’.

Carolyn Wells

Banish Mavis and Connect to the Future

I wrote a couple of days ago about making this blog accessible over IPv6. Most of my readers probably shrugged a gallic shrug and muttered ‘À chacun son goût‘ before moving on to more exciting things like emptying the vacuum cleaner. But wait! This stuff is actually interesting and important, and it will affect you, so here’s my attempt to explain what it’s all about…

You probably know the basics. Machines on the internet have an address which looks like – those four numbers represent four bytes, and while four bytes provide rather a large number of addresses, it turns out that this internet thing is rather popular, and four bytes isn’t nearly enough. These addresses have been in short supply for some time, and so, while your broadband router will have one, the laptops, XBoxes, smartphones, etc on your local network can’t all get an address of their own. Your heating controller, your TV, your electricity meter will soon all want to be connected too. No way, Jose.

Instead, most networked devices use addresses that are only valid on your local network – a reserved group beginning 192.168… Think of 192.168 addresses as internal phone extensions, rather than proper phone numbers. Whenever your devices want to communicate with the outside world, they do so via a special bit of software on your router called ‘NAT’ – the ‘Network Address Translation’ system – that temporarily connects 192.168 internal addresses to real addresses in the outside world. It’s a bit like a telephone switchboard operator – let’s call her Mavis – who can connect internal phone extensions to a limited number of external lines.

In fact, we can push the switchboard analogy a bit further because, in general, the machines on your network can contact the outside world automatically by the equivalent of dialling ‘9’ to get an outside line. The outside world, however, can’t contact you without going through Mavis, and she’s very picky about who she’ll let through. There are no direct-dial numbers. This has some real benefits: it can keep pesky salesmen at bay. But it also makes it really hard for your spouse/lover/next-employer to get in touch with you for more delightful discussions. Mavis gets rather too involved in those… And remember, we’re not just talking about the office, here. Mavis is also in your sitting room at home.

The interesting thing is that we’ve been in this situation for so long that we accept it as normal. Most of us have had NAT for as long as we’ve had broadband connections, and it has fundamentally affected the way we think about the internet and what it can do for us. We’ll come back to that in a minute.

Despite all of the above, we might have gone on employing Mavis for quite a while, if it weren’t for a much bigger problem looming on the horizon: we’re running out of addresses even to give to the routers. Not much point in having an efficient switchboard operator if there are no outside lines! Your ISP has to manage the IP addresses it hands out carefully and make sure there aren’t too many unused ones lying around. If your internet connection uses DSL, for example, you may find that your router’s IP address – the phone number of your outside line – changes fairly regularly as a result, so there’s little chance of your spouse/lover/next-employer being able to talk to Mavis even if they wanted to!

This is the basic problem that IPv6 is designed to fix. In the future, internet addresses will be made up not of 4 bytes, but of 16, which gives a wonderfully huge number of addresses, and every connected device in the world can have one (or indeed, several, which often proves useful). IPv6 has some other useful features too, but this is the most important.

A quick aside – what do these new addresses look like?

IPv6 addresses are the equivalent of 39-digit phone numbers, so they’re less convenient if you actually have to type them in! To make them a bit more manageable, they are written as eight groups of 4 hexadecimal digits, which makes them look a bit strange and scary to anyone who’s just getting used to addresses like

In reality, most people will almost never have to type, or even see, one, but I know that Status-Q readers are a smart and intelligent bunch, so to satisfy your curiosity, here’s an IPv6 address:


By convention, when writing these, you can leave out any leading zeros in the groups, and also any single sequence of groups consisting entirely of zeros, so the above can be abbreviated to:


There you are. Now you can recognise IPv6 addresses when you see them, but you’d probably much rather deal with

which resolves to the same thing: the DNS can hand out IPv6 addresses in just the same way as the old IPv4 ones.

One last thing before we get back to the big-picture stuff: there are some address groups set aside for specific uses. Addresses beginning fe80::, for example, are the equivalent of 192.168 addresses – they are just designed to work on the local network. A device will have at least one of these automatically, as well as any global addresses it may have been allocated.

In fact, if you have a reasonably recent operating system, you’ve probably got one already on the machine you’re using now, though you may need to burrow a bit into your network configuration to find it. (On a Mac or Linux box, run ‘ifconfig‘ on the command line. On Windows, you need to enable IPv6 first and then use ‘ipconfig‘). So you can already use utilities like ‘ssh’ and ‘ping6’ to talk to other machines on your network using these addresses, even if not to the outside world.

I’ll do a more technical post about this soon for those who are interested. But for now…

Why is all this important?

Here are some things that we’ve just come to assume are a bit difficult:

  • Backing up your computer to a hard drive on your sister’s machine, and allowing her to do the same to yours.
  • Plugging in a networked webcam and accessing it from anywhere.
  • Printing something on your office printer when you’re at home, or on your home printer when you’re at Starbucks.
  • Logging in to your parents’ machine to help sort out a problem.
  • Accessing that presentation you left on your machine at home from the conference centre where you’re supposed to be giving a talk in the morning.
  • Making VoIP (internet-based telephone systems) work reliably.
  • Running your own web server on a computer at home.
  • Turning on your heating from the airport as you come home from holiday.

All of these things can of course be done at present, but they usually involve special expertise, or services like Dropbox, GoToMeeting, DynDNS or Skype, which have grown up largely to deal with the fact that, in general, you can’t just connect to other machines on the internet because they don’t have a globally-accessible address. Sometimes it’s a case of going and having a polite but firm word with Mavis to say that she really must allow certain calls through. In other cases, it’s much more complex. The electricity company could easily put a smart meter in your house, for example, so they don’t have to bother you so often, but they’d have to speak to every Mavis in the country. A daunting task, I think you’ll agree. And then there are other systems like uPnP that are used by games consoles, for example, to sneak behind Mavis’s back and do a little wiring of their own just to make it easy to play games with your friends. All rather messy.

In the early days, you could assume that two machines which were connected to the internet and switched on could just talk to each other. All sorts of things were much easier back then. Now, there may, of course, be many reasons why you don’t want people to be able to connect to all of your devices, even if they have passwords and firewalls, but the important point is that this should be something that you can decide: it should be a question of policy, not one of capability. At the moment, you generally can’t get an internet connection at all without employing Mavis, and that just makes everything so much more difficult. But when everything is connected by IPv6, a lot of things at least become possible, maybe even easy. (And a lot of businesses may have to think about their business models.)

This is why you want to encourage your ISP to support IPv6 as soon as they can. They’ve known about it for a long time, and it’s been rigorously tested. In a future post, I’ll write about how you can bypass both Mavis and your ISP if you want to start using this now. But at least, I hope, you understand why it is more important than simply a change of phone number.


All that’s best of dark and bright

Thetford Forest this afternoon


Visiting the Library

A tip for Mac users: The Library folder in your home directory contains miscellaneous preference files, support libraries etc, and most normal users seldom need to touch it, which is why it is hidden by default in Mac OS X Lion. The standard way to get to it, if needed, is to hold down Option while clicking the ‘Go’ menu in Finder, and it’ll appear among the list of destinations.

If, however, you’re not a ‘normal user’ you may wish to make it more visible, and this turns out to be very easy. Just open a Terminal window and type:

chflags nohidden ~/Library

And you should then see it in your Finder just like any other folder.

Thanks to MOApp for this one.

Future-proofing my blog

Well, IPv6 has now been officially launched on the world, and this is a quick note to let you networking geeks know that Status-Q is now IPv6-enabled. You may be connecting that way already, and in theory you shouldn’t be able to tell, because has both an IPv4 and IPv6 address and your browser will use whichever one it prefers. I’m going to write a bit more about IPv6 soon.

But if you want to check whether you have IPv6 connectivity to here, you can try going to

which is an IPv6-only address.

It probably won’t work yet for most of you. UK ISPs are being a bit slow on the uptake and, to be fair, with some reason: it’s a big change. So if you want to get IPv6 connectivity at home you may need to jump through quite a few technical hoops (and I’ll write soon about how I did it). But they’ll have to support it eventually, and most modern operating systems handle IPv6 out-of-the-box and have done for some time, so once the connectivity is there, everything just works.

A very handy site, if you’re playing with this is

which will tell you what your connection and browser can do.

More on this topic soon.

Raspberry Pi Webcam Viewer

I finally got a chance to play with my RaspberryPi, so I threw together a quick experiment.



Update: A few people have asked me for a little more information. I’m happy to make the source code available, but it’s not very tidy and a bit specific to my situation… however, to answer some of the questions:

The enclosure for the Raspberry Pi comes from SK Pang Electronics, and it’s perfect for my needs. You can buy just the perspex cover, but they do a nice Starter Kit which includes the breadboard, some LEDs, resistors and the pushswitch. Definitely recommended.

For the graphics, I used the PyGame library, which has the advantage of being cross-platform: you can use it with a variety of different graphics systems on a variety of different devices. On most Linux boxes, you’d normally run it under X Windows, but I discovered that it has various drivers that can use the console framebuffer device directly. This makes for a quicker startup and lighter-weight system, though I imagine it probably has less access to hardware acceleration, so it’s probably not the way to go if your graphics need high performance. You can read about how to get a PyGame display ‘surface’ (something you can draw on) from the framebuffer, in a handy post here.

To load an image from a file in PyGame is easy: you do something like this:

            im_surf = pygame.image.load(f, "cam.jpg")

where ‘f’ is an open file, and the ‘cam.jpg’ is just an invented filename to give the library a hint about the type of file it’s loading.

Now, with a webcam, we need to get the image from a URL, not from a file. It’s easy to read the contents of a URL in Python. You just need something like:

            import urllib
            img = urllib.urlopen(img_url).read()

but that will give you the bytes of the image as a string. If we want to convert it into a PyGame surface, we need to make it look more like a file. Fortunately, Python has a module called StringIO which does just that: allows you to treat strings as if they were files. So to load a JPEG from img_url and turn it into a PyGame surface which you can blit onto the screen, you can do something like:

          f = StringIO.StringIO(urllib.urlopen(img_url).read())
          im_surf = pygame.image.load(f, "cam.jpg")

I’ll leave the remaining bits as an exercise for the reader!

If you like this, you might also like my CloudSwitch

Disk Risk

Mmm. I seem to have had a lot of hard drive failures recently – Seagate drives, mostly, though, to be fair, the majority of my drives are Seagate just because my favourite supplier happens to like them, so I would expect see more failures there. The last one, though, is just 18 months old and has started making ominous clicking noises. They don’t make ’em like they used to. Stuff I’ve read online tends to suggest that it’s hard to assign blame to particular drive manufacturers, but particular models do tend to have rather different failure rates.

I do, I realise, have rather a lot of hard disks. I have three 4-bay Drobo enclosures, for a start, so that’s 12 drives even before I start adding on the miscellaneous backup disks, TV-recording disks, etc. Not to mention the internal ones in all our various machines. There must be 20-25 hard disks around here, and even though manufacturers’ specs talk about a <1% annual failure rate, studies tend to suggest that real-world figures are rather higher. One of the biggest studies, done by Google a few years ago, showed failure rates of 1.7% in the first year, rising to over 8% in the third year.

Yes, many of my drives are about that age, so if I really have 25 of them, I guess I should expect one to die every six months or so. Bother.

This suggests to me that money spent on things like my Drobo enclosures is worthwhile, because, though they are pricey, especially once you’ve filled them up with drives, any single drive failure is unlikely to be catastrophic – as disks die, you just replace them with whatever size is currently in vogue. My main Drobo currently has two 2TB drives, one 1.5TB, and a 1TB. There are those, I know, who have had less positive experiences with some Drobo kit – I found a DroboShare networking add-on to be decidedly wobbly at a past company – but in the simple use case of a Drobo plugged into a computer, I’ve been very happy and have replaced several drives without ever losing data.

The other thing that the Google study found was a strong correlation between when disks start reporting errors (which they can do using the S.M.A.R.T technology built into modern drives) and a failure soon afterwards. It’s worth, therefore, having something that checks the S.M.A.R.T status and lets you know about issues as soon as they are reported, even if the drive is still apparently working OK. On the Mac, Disk Utility can tell you about issues, but only when you go and look, so I use SMARTreporter to give more regular checks.

OK, things are getting better. There is another issue, though.

On the Mac, at least, most external drives are connected by USB or Firewire, and in general S.M.A.R.T information is not read through those interfaces – if you look in Disk Utility, you’ll see it’s ‘Unavailable’. More sophisticated enclosures like the Drobo will check the S.M.A.R.T status themselves and warn you when things look dubious, but your average USB-connected backup drive may give you no such warnings.

So I was interested to discover this kernel driver project which enhances the standard OSX USB and FireWire drivers to make S.M.A.R.T available for a lot more interfaces. (Download v0.5 here). I’ll try it on my Media Mac Mini, which has three external drives, and see how it goes…

Remote keyboard and mouse for your RaspberryPi

Warning – technical post ahead…

My shiny new RaspberryPi came through the door this week, and the first thing I noticed was that it was the first computer I’d ever received where the postman didn’t need to ring the bell to deliver it.

The next thing I discovered, because it came sooner than expected, was that I was missing some of the key bits needed to play with it. A power supply was no problem – I have a selection of those from old phones and things, and I found a USB-to-micro-USB cable. Nor was a network connection tricky – I have about as many ethernet switches as I have rooms in the house.

A monitor was a bit more challenging – I have lots of them about, but not with HDMI inputs, and I didn’t have an HDMI-DVI adaptor. But then I remembered that I do have an HDMI monitor – a little 7″ Lilliput – which has proved to be handy on all sorts of occasions.

The next hiccup was an SD card. You need a 2GB or larger one on which to load an image of the standard Debian operating system. I had one of those, or at least I thought I did. But it turned out that my no-name generic 2GB card was in fact something like 1.999GB, so the image didn’t quite fit. And a truncated filesystem is not the best place to start. But once replaced with a Kingston one – thanks, Richard – all was well.

Now, what about a keyboard and mouse?

Well, a keyboard wasn’t a problem, but it didn’t seem to like my mouse. Actually, I think the combined power consumption probably just exceeded the capabilities of my old Blackberry power supply, which only delivers 0.5A.

If you’re just doing text-console stuff, then this isn’t an issue, because it’s easy to log into it over the network from another machine. It prints its IP address just above the login prompt, so you can connect to it using SSH, if you’re on a real computer, or using PuTTY if you’re on Windows.

But suppose you’d like to play with the graphical interface, even though you don’t have a spare keyboard and mouse handy?

Well, fortunately, the Pi uses X Windows, and one of the cunning things about X is that it’s a networked display system, so you can run programs on one machine and display them on another. You can also send keyboard and mouse events from one machine to another, so you can get your desktop machine to send mouse movements and key presses from there. On another Linux box, you can run x2x. On a Mac, there’s Digital Flapjack’s osx2x (If that link is dead, see the note at the end of the post).

These both have the effect of allowing you to move your mouse pointer off the side of the screen and onto your RaspberryPi. If you have a Windows machine, I don’t think there’s a direct equivalent. (Anyone?) So you may need to set up something like Synergy, which should also work fine, but is a different procedure from that listed below. The following requires you to make some changes to the configurations on your RaspberryPi, but not to install any new software on it.

Now, obviously, allowing other machines to interfere with your display over the network is something you normally don’t want to happen! So most machines running X have various permission controls in place, and the RaspberryPi is no exception. I’m assuming here that you’re on a network behind a firewall/router and you can be a bit more relaxed about this for the purposes of experimentation, so we’re going to turn most of them off.

Running startx

Firstly, when you log in to the Pi, you’re normally at the command prompt, and you fire up the graphical environment by typing startx. Only a user logged in at the console is allowed to do that. If you’d like to be able to start it up when you’re logged in through an ssh connection, you need to edit the file /etc/X11/Xwrapper.config, e.g. using:

sudo nano /etc/X11/Xwrapper.config

and change the line that says:


to say:


Then you can type startx when you’re logged in remotely. Or startx &, if you’d like it to run in the background and give you your console back.

Allowing network connections

Secondly, the Pi isn’t normally listening for X events coming over the network, just from the local machine. You can fix that by editing /etc/X11/xinit/xserverrc and finding the line that says:

exec /usr/bin/X -nolisten tcp "$@"

Take out the ‘-nolisten tcp’ bit, so it says

exec /usr/bin/X "$@" 

Now it’s a networked display system.

Choosing who’s allowed to connect

There are various complicated, sophisticated and secure ways of enabling only very specific users or machines to connect. If you want to use those, then you need to go away and read about xauth.

I’m going to assume that on your home network you’re happy for anyone who can contact your Pi to send it stuff, so we’ll use the simplest case where we allow everything. You need to run the ‘xhost +‘ command, and you need to do it from within the X environment once it has started up.

The easiest way to do this is to create a script which is run as part of the X startup process: create a new file called, say:


It only needs to contain one line:

/usr/bin/xhost +

Now, when the graphical environment starts up, it will allow X connections across the network from any machine behind your firewall. This lets other computers send keyboard and mouse events, but also do much more. The clock in the photo above, for example, is displayed on my Pi, but actually running on my Mac… however, that’s a different story.

For now, you need to configure x2x, osx2x, or whatever is sending the events to send them to ip_address:0, where ip_address is the address of your Pi. I’m using osx2x, and so I create the connection using the following:

Once that’s done, I can just move the mouse off the west (left-hand) side of my screen and it starts moving the pointer on the RaspberryPi. Keyboard events follow the pointer. (I had to click the mouse once initially to get the connection to wake up.)

Very handy, and it saves on desk space, too!

Update: Michael’s no longer actively maintaining binaries for osx2x, and the older ones you find around may not work on recent versions of OS X. So I’ve compiled a binary for Lion(10.7) and later, which you can find in a ZIP file here. Michael’s source code is on Github, and my fork, from which this is built, is here.

© Copyright Quentin Stafford-Fraser